1. Technical Field
This invention relates to cordless telephones and, more particularly, to a cordless telephone having a plurality of portable units arranged for communicating with a base unit.
2. Description of the Prior Art
Cordless telephone systems presently available in the art include multiple portable units that operate with a single base unit. An example of such a telephone system is described in U.S. Pat. No. 5,325,420. Although such cordless telephone systems exist in the art, the number of portable units that operate easily with a single base is generally limited, typically to two or three units. This is due primarily to a widely used architecture that requires the base unit to keep track of the number of portable units in the system.
The cordless telephone systems employing multiple portable units for operation with a single base are required to somehow synchronize all portable units with the base unit on a common channel from a plurality of channels for setting up calls, i.e., initiating communications to and receiving communications from the base unit. Otherwise, by way of example, an incoming ringing signal received at the base unit and retransmitted to the portable units may not be received by all of the portable units. Rather, this ringing signal will be received only by the portable unit that is on the channel last used by the base unit, if for some reason, the base unit and this portable unit moved away from the common channel. Other portable units in the system are thus not able to reach and communicate with the base unit.
One cordless telephone system available in the art employs a dedicated common signaling channel to set up calls. Communication is initiated on this dedicated channel and the base unit and the portable unit involved in the communication both move to another suitable channel for voice communication. Since cordless telephones have been allocated a limited number of communication channels over which they may operate, however, this system has the disadvantage of further reducing the available number of channels by minimally committing one of these as the common signaling channel.
Another cordless telephone system, in executing a channel synchronization process, requires each portable unit to confirm that it has received the latest or last current channel. This confirmation process limits the number of portable units that reasonable can be in the system, however, since every time a channel change takes place each portable unit in the system must respond. Thus the response time is appreciably degraded as additional portable units are added to this system.
Although the above-described cordless telephone systems wherein multiple portable units are able to communicate with a single base do provide cordless telephone communications, they do so only with the described limitations. It is desirable therefore to have a cordless telephone wherein multiple portable units communicate with a single base without these limitations.